Bleaching agents generally used as laundry aids fall within two major categories: chlorine releasing and oxygen releasing compounds. Oxygen releasing bleaches are preferred since chlorine releasing bleaches are harsh to fabrics and fabric dyes and cause yellowing of many synthetic resin fibers. Oxygen releasing bleaches include hydrogen peroxide, compounds which release hydrogen peroxide in aqueous solutions and true per acids and their salts. Solid, oxygen-releasing bleaches are generally used both as bleach additives in laundry detergents and as the active ingredient in dry formulated laundry bleaches.
Detergent powders often contain hydrogen peroxide releasing compounds such as sodium perborate, sodium percarbonate, and the like in order to assist in the brightening and bleaching of textiles during washing. Sodium perborate is normally preferred as a solid bleach additive because it is more stable than percarbonate in detergent powders during prolonged storage. External moisture, the alkalinity of the detergent, the presence of different components in a detergent mixture and the free moisture content of the detergent contribute to the instability of sodium percarbonate in detergent compositions. Thus, percarbonates which are otherwise inexpensive, safe and effective bleaching agents, have not found widespread use in laundry detergents and other washing compositions.
Since the loss of hydrogen peroxide or the active oxygen content of sodium percarbonate occurs at a faster rate under humid conditions, several water resistant additives have been suggested as percarbonate stabilizers. Such additives which may impart moisture resistance to percarbonate are long chain fatty acids, their esters and amides, copolymer and wax emulsions and the like. To show maximum effectiveness, such hydrophobic additives should be added in relatively high concentrations to percarbonate. However, when such products are added to water, as in laundry wash, an undesirable insoluble residue remains behind.
Chemical stabilization of sodium percarbonate has been attempted by several workers in the field. Among the chemical stabilizers that improve sodium percarbonate stability most are magnesium compounds and sodium silicate. The preferred stabilizer for sodium percarbonate has been magnesium silicate which is incorporated in sodium percarbonate during its preparation from sodium carbonate and hydrogen peroxide.
German Pat. No. 870,092 discloses the use of an aerosol of fumed silica to stabilize percarbonate which may be used in bleaching or washing compositions. However, the fumed silica of that reference is aggregated in form, having particle sizes on the order of 2 to 10 microns. These particles have only a partially hydrated structure and are not dispersible in water to yield low viscosity, high concentration sols without the application of sufficiently high shear forces to disaggregate the particles, thereby causing the disaggregated surfaces to hydrolyze and eventually disperse. Not only are such procedures time consuming and cumbersome but they are costly as well, involving the application of excessively high quantities of energy to mechanically initiate the necessary chemical reaction (hydrolysis) to achieve dispersion. These considerations are discussed by K. A. Loftman in the publication "Ultrafine Particles," edited by W. E. Kuhn, published by John Wiley and Sons, Inc., (N.Y.) (1963), pp. 203-204, and by R. K. Iler, in the publication "Surface and Colloid Science," Vol. 6, edited by E. Matijevic, published by John Wiley and Sons, Inc., (1973), pp. 20 and 39-65.